Does he put his ears back, try to bite out, kick out when you put it on? These are all signs that he doesn't like the saddle.
Make sure it is behind scapula and in front of the last rib. Most people sit their saddles over top of the shoulders which restricts the movement in this area.
Is it too high or low in front (can be too wide or too narrow). You need to imagine a marble on the seat and where it would stop if it was rolled on there. Would it go more towards the pommel or the cantle or would it be fairly centered? Stand back and have a good look. It needs to be taken into consideration if the horse has muscle wastage and/or postural problems when looking at how the saddle sits.
No bridging or rocking. If you can see daylight in under the centre of the panels, it is bridging.
Is there clearance all the way to the base of the wither? This is a common area for horses to get pinched. If you put your hand either side of the wither and gently rock saddle side to side is it pinching your fingers? Can you reach down to the base of the withers? You should also be able to do this when you are sitting in the saddle.
When you push down on the pommel does the back of the saddle lift up? This can be that the saddle is too wide or lack of panel stuffing or the wrong style for your horse. If your horse has muscle wastage that can also cause the saddle to tip.
This time when you push down on the pommel can you run your fingers down the front of the saddle (that goes down behind the shoulder)? If you can do this when it's just sitting on the horse, there is no way you will be able to do it when there is a rider on it as well.
See the talc test instructions page.